Transponder for a radio-over-fiber optical fiber cable

ABSTRACT

The invention is a transponder for a radio-over-fiber (RoF) optical fiber cable. The transponder includes a converter unit made up of an electrical-to-optical (E/O) converter and an optical-to-electrical (O/E) converter. The optical fiber cable optically couples the converter unit to a head-end unit that sends and receives optical RF signals. A dipole antenna system is operably coupled to the converter unit and is arranged so as to create elongate picocell in a direction perpendicular to the optical fiber cable when the transponder is in communication with the head-end unit. The asymmetric picocell shape allows for creating a picocellular coverage area using fewer optical fiber cables than is possible with prior art transponders.

FIELD OF THE INVENTION

The present invention relates generally to radio-over-fiber (RoF)systems, and in particular relates to transponders for a RoF opticalfiber cable used in RoF systems.

BACKGROUND OF THE INVENTION

Wireless communication is rapidly growing, with ever-increasing demandsfor high-speed mobile data communication. As an example, so-called“wireless fidelity” or “WiFi” systems and wireless local area networks(WLANs) are being deployed in many different types of areas (coffeeshops, airports, hospitals, libraries, etc.). The typical wirelesscommunication system has a head-end station connected to an access pointdevice via a wire cable. The access point device includes a RFtransmitter/receiver operably connected to an antenna, and digitalinformation processing electronics. The access point device communicateswith wireless devices called “clients,” which must reside within thewireless range or a “cell coverage area” in order to communicate withthe access point device.

The size of a given cell is determined by the amount of RF powertransmitted by the access point device, the receiver sensitivity,antenna gain and the RF environment, as well as by the RFtransmitter/receiver sensitivity of the wireless client device. Clientdevices usually have a fixed RF receive sensitivity, so that theabove-mentioned properties of the access point device largely determinethe cell size. Connecting a number of access point devices to thehead-end controller creates an array of cells that cover an area calleda “cellular coverage area.”

One approach to deploying a wireless communication system involvescreating “picocells,” which are wireless cells having a radius in therange from about a few meters up to about 20 meters. Because a picocellcovers a small area, there are typically only a few users (clients) perpicocell. A closely packed picocellular array provides high per-userdata-throughput over the picocellular coverage area. Picocells alsoallow for selective wireless coverage in small regions that otherwisewould have poor signal strength when covered by larger cells created byconventional base stations.

One type of wireless system for creating picocells utilizesradio-frequency (RF) signals sent over optical fibers—called “radio overfiber” or “RoF” for short. Such systems include a head-end unitoptically coupled to a transponder via an optical fiber link. Unlike aconventional access point device, the transponder has no digitalinformation processing capability. Rather, the digital processingcapability resides in the head-end unit. The transponder is transparentto the RF signals and simply converts incoming optical signals from theoptical fiber link to electrical signals, which are then converted toelectromagnetic signals via an antenna. The antenna also receiveselectromagnetic signals and converts them to electrical signals. Thetransponder then converts the electrical signals to optical signals,which are then sent to the head-end unit via the optical fiber link.

The transponders are typically included in an optical fiber cable thatincludes the optical fiber links for each transponder. The picocellsassociated with the distributed transponders form a picocell coveragearea. To reduce picocell cross-talk, high-directivity transponderantennas can be used. Their use, however, requires additional efforts atthe manufacturing and installation stages because proper adjustment andorientation of each antenna is necessary. Installing multiple directiveantennas per transponder (e.g., to support both data and voice servicesin different frequency bands) further complicates installation andimposes tight requirements for integration of antennas with thetransponder. In addition, the size and orientation of the picocellsrequires direct adjustment of the antennas, which is difficult to doonce the antennas are incorporated into the optical fiber cable.

SUMMARY OF THE INVENTION

One aspect of the invention is a transponder for a radio-over-fiber(RoF) optical fiber cable. The transponder includes anelectrical-to-optical (E/O) converter and an optical-to-electrical (O/E)converter. The system also includes a dipole antenna system operablycoupled to the E/O converter and the O/E converter. The antenna systemis arranged relative to the optical fiber cable so as to create anelongate picocell in a direction locally perpendicular to the opticalfiber cable when the transponder is addressed.

Another aspect of the invention is a RoF picocellular wireless system.The system includes a head-end unit adapted to send and receive opticalRF signals. The system also includes one or more transponders of thetype described immediately above. The system further includes one ormore optical fiber cables that include the one or more transponders andthat optically couple the head-end unit to each transponder.

Another aspect of the invention is a method of forming an elongatepicocell for a RoF system. The method includes transmitting optical RFsignals to a transponder via a downlink optical fiber in the opticalfiber cable, and converting the optical signals to electrical RF signalsat the transponder. The method also includes converting the electricalsignals to electromagnetic RF signals at the transponder using a dipoleantenna system to create the elongate picocell in a direction locallyperpendicular to the optical fiber cable.

Additional features and advantages of the invention are set forth in thedetailed description that follows, and will be readily apparent to thoseskilled in the art from that description or recognized by practicing theinvention as described herein, including the detailed description thatfollows, the claims, as well as the appended drawings.

It is to be understood that both the foregoing general description andthe following detailed description present embodiments of the invention,and are intended to provide an overview or framework for understandingthe nature and character of the invention as it is claimed. Theaccompanying drawings are included to provide a further understanding ofthe invention, and are incorporated into and constitute a part of thisspecification. The drawings illustrate various embodiments of theinvention and, together with the description, serve to explain theprinciples and operations of the invention.

Accordingly, various basic electronic circuit elements andsignal-conditioning components, such as bias tees, RF filters,amplifiers, power dividers, etc., are not all shown in the Figures forease of explanation and illustration. The application of such basicelectronic circuit elements and components to the present invention willbe apparent to one skilled in the art.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a generalized embodiment of a RoFpicocellular wireless system according to the present invention;

FIG. 2 is a schematic close-up view of an example embodiment of the RoFsystem of FIG. 1, illustrating an example embodiment of the converterunit and dipole antenna system for the transponder of the presentinvention as arranged in the optical fiber cable;

FIG. 3 is a close-up schematic diagram of an example embodiment of theconfiguration of the converter unit and dipole antenna system for thetransponder of the present invention, wherein the dipole antenna systemincludes transmitting and receiving antennas;

FIG. 4 is a schematic diagram of an example embodiment of thetransponder of the present invention similar to that shown in FIG. 3,but wherein dipole antenna system includes a single antenna that bothtransmits and receives electromagnetic signals;

FIG. 5 is a close-up schematic diagram of an example embodiment of anE/O converter in the converter unit that includes a number ofamplifiers, with each amplifier adapted to amplify a different frequencyin the electrical RF signal created by the photodetector;

FIG. 6 is a schematic diagram of an example embodiment of thetransponder of the present invention, wherein the dipole antenna systemincludes a power divider and three separate antennas;

FIG. 7 is a schematic diagram of an example embodiment of thetransponder of the present invention similar to that shown in FIG. 6,wherein the antenna system includes a plurality of power dividers eachelectrically coupled to an antenna;

FIG. 8 is a schematic diagram of an example embodiment of thetransponder according to the present invention similar to that shown inFIG. 7, wherein a portion of the antenna system lies outside of theoptical fiber cable coating;

FIG. 9 is a schematic diagram of an example embodiment of thetransponder of the present invention similar to that shown in FIG. 8,wherein the converter unit and dipole antenna system both reside outsideof the optical fiber cable coating;

FIG. 10 is a schematic diagram of an example embodiment of thetransponder of the present invention, wherein the antenna systemincludes two pairs of wire antennas, with each antenna connected to theconverter unit via respective RF cable sections;

FIG. 11 is a schematic diagram of an example embodiment of the RoFpicocellular wireless system of FIG. 1, showing details of an exampleembodiment of the head-end unit;

FIG. 12 is a schematic diagram of a typical prior art picocellularcoverage area formed by a conventional prior art picocellular wirelesssystem that employs conventional omnidirectional transponders;

FIG. 13 is a schematic diagram of an example picocellular coverage areaformed by the RoF picocellular wireless system of the present inventionthat utilizes the transponders of the present invention; and

FIG. 14 is a plot of RF power (dBm) emitted by the dipole antenna systemof the transponder of the present invention vs. the distance (m) fromthe antenna system along both the x-direction (i.e., perpendicular tothe optical fiber cable) and the y-direction (i.e., along the opticalfiber cable).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference is now made in detail to the present preferred embodiments ofthe invention, examples of which are illustrated in the accompanyingdrawings. Whenever possible, the same or analogous reference numbers areused throughout the drawings to refer to the same or like parts.

Generalized Picocellular Wireless System with Transponder

FIG. 1 is a schematic diagram of a generalized embodiment of a RoFpicocellular wireless system 10 according to the present invention.System 10 includes a head-end unit 20 adapted to transmit, receiveand/or process RF optical signals. In an example embodiment, head-endunit 20 is operably coupled to an outside network 24 via a network link25, and the head-end unit serves as a pass-through for RF signals sentto and from the outside network.

System 10 also includes one or more transponder units (“transponders”)30 according to the present invention. Each transponder 30 includes aconverter unit 31 and a dipole antenna system 32 electrically coupledthereto, wherein the dipole antenna system has a dipole radiationcharacteristic the same as or substantially similar to that of an idealdipole antenna. Transponder 30 is discussed in greater detail below.

System 10 includes one or more optical fiber cables 34 each opticallycoupled to head-end unit 20. Each optical fiber cable 34 includes one ormore optical fiber RF transmission links 36 optically coupled torespective one or more transponders 30. In an example embodiment, eachoptical fiber RF transmission link 36 includes a downlink optical fiber36D and an uplink optical fiber 36U. Example embodiments of system 10include either single-mode optical fiber or multimode optical fiber fordownlink and uplink optical fibers 36D and 36U. The particular type ofoptical fiber depends on the application of system 10, as well as thedesired performance and cost considerations. For many in-buildingdeployment applications, maximum transmission distances typically do notexceed 300 meters. The maximum length for the intended RoF transmissionneeds to be taken into account when considering using multi-mode opticalfibers for downlink and uplink optical fibers 36D and 36U. For example,it has been shown that a 1400 MHz.km multi-mode fiber bandwidth-distanceproduct is sufficient for 5.2 GHz transmission up to 300 meters. In anexample embodiment, the present invention employs 50 μm multi-modeoptical fiber for the downlink and uplink optical fibers 36D and 36U,and E/O converters (introduced below) that operate at 850 nm usingcommercially available vertical-cavity surface-emitting lasers (VCSELs)specified for 10 Gb/s data transmission.

In an example embodiment, RoF picocellular wireless system 10 of thepresent invention employs a known telecommunications wavelength, such as850 nm, 1300 nm, or 1550 nm. In another example embodiment, system 10employs other less common but suitable wavelengths such as 980 nm.

Also shown in FIG. 1 is a local x-y-z Cartesian coordinate system C ateach dipole antenna system 32 for the sake of reference, where thex-direction is into the paper and locally perpendicular to optical fibercable 34. In an example embodiment, dipole antenna system 32 issufficiently stiff so that optical fiber cable 34 is locally straight atthe dipole antenna system location. In an example embodiment, dipoleantenna system 32 is located relatively far away from converter unit 31(e.g., 2 meters), while in other example embodiments the dipole antennasystem is located relatively close to the converter unit (e.g., a fewinches away), or even directly at the converter unit.

Each transponder 30 is adapted to form a picocell 40 via dipole antennasystem 32 via electromagnetic transmission and reception when thetransponder is addressed, e.g., receives a downlink optical signal SD′from head-end unit 20 and/or an uplink electromagnetic signal SU″ from aclient device 46. Client device 46, which is shown in the form of acomputer as one example of a client device, includes an antenna 48(e.g., a wireless card) adapted to electromagnetically communicate with(i.e., address) the transponder and antenna system 32 thereof.

Dipole antenna system 32 is adapted to form picocell 40 from a dipoleradiation pattern 42 oriented perpendicular to optical fiber cable 34 atthe location of the dipole antenna system. The term “locallyperpendicular” is used herein to describe the orientation of picocell 40and/or the corresponding dipole radiation pattern 42 relative to opticalfiber cable 34 at the location of dipole antenna system 32. Dipoleradiation pattern 42 is thus centered about the local x-z plane P_(XZ)(viewed edge-on in FIG. 1 and illustrated as a dotted line). In anexample embodiment, only a portion of dipole radiation pattern 42 isused for picocell 40, e.g., the portion below optical fiber cable 34(i.e., in the -z direction), as shown in FIG. 1.

In an example embodiment, system 10 is powered by a power supply 50electrically coupled to head-end unit 20 via an electrical power line 52that carries electrical power signals 54.

Transponder Incorporated Into Optical Fiber Cable

FIG. 2 is a schematic close-up view of an example embodiment oftransponder 30 as incorporated into optical fiber cable 34. In anexample embodiment, optical fiber cable 34 includes an outer coating 58.As mentioned above, transponder 30 includes a converter unit 31.Converter unit 31 includes an electrical-to-optical (E/O) converter 60adapted to convert an electrical signal into a corresponding opticalsignal, and an optical-to-electrical (O/E) converter 62 that converts anoptical signal into a corresponding electrical signal. E/O converter 60is optically coupled to an input end 70 of uplink optical fiber 36U andO/E converter 62 is optically coupled to an output end 72 of downlinkoptical fiber 36D.

In an example embodiment, optical fiber cable 34 includes electricalpower line 52, and converter unit 31 includes a DC power converter 80electrically coupled to the electrical power line and to E/O converter60 and O/E converter 62. DC power converter 80 is adapted to change thevoltage levels and provide the power required by the power-consumingcomponents in converter unit 31. In an example embodiment, DC powerconverter 80 is either a DC/DC power converter, or an AC/DC powerconverter, depending on the type of power signal 54 carried byelectrical power line 52. In an example embodiment, electrical powerline 52 includes two electrical wires 52A and 52B connected to DC powerconverter 80.

As discussed above, dipole antenna system 32 is electrically coupled toconverter unit 31. In an example embodiment, dipole antenna system 32includes one or more antenna elements (“antennas”) 33. In the exampleembodiment shown in FIG. 2, antenna system 32 includes a receivingantenna 33R electrically coupled to E/O converter 60 via a first RFcable section 90 and a transmitting antenna 33T electrically coupled toE/O converter 62 via a second RF cable section 90. In an exampleembodiment, the one or more antennas 33 is/are made of or includesections of wire. One or more RF cable sections 90 are used in exampleembodiments of the present invention to connect corresponding one ormore antennas 33 to the converter unit 31. In an example embodiment,dipole antenna system 32 supports multiple frequency bands.Additionally, the diversity principle can be used to send the sameinformation through statistically independent channels.

Example Transponder Converter Unit

FIG. 3 is a detailed schematic diagram of an example embodiment ofconverter unit 31 and dipole antenna system 32 for the transponder 30 ofthe present invention. In the example embodiment of FIG. 3, E/Oconverter 60 includes a laser 100 optically coupled to an input end 70of uplink optical fiber 36U, a bias-T unit 106 electrically coupled tothe laser, an amplifier 110 electrically coupled to the bias-T unit, anda RF filter 114 electrically coupled to the amplifier and to thecorresponding RF cable section 90. Also in an example embodiment, O/Econverter 62 includes a photodetector 120 optically coupled to outputend 72 of downlink optical fiber 36D, an amplifier 110 electricallycoupled to the photodetector, and a RF filter 114 electrically coupledto the amplifier and to the corresponding RF cable section 90. In anexample embodiment, laser 100 is adapted to deliver sufficient dynamicrange for one or more RoF applications. Examples of suitable lasers forE/O converter 60 include laser diodes, distributed feedback (DFB)lasers, Fabry-Perot (FP) lasers, and VCSELs.

In the operation of transponder 30 of FIG. 3, a downlink optical signalSD′ traveling in downlink optical fiber 36D exits this optical fiber atoutput end 72 and is received by photodetector 120. Photodetector 120converts optical signal SD′ into a corresponding electrical signal SD,which is then amplified by amplifier 110 and then filtered by RF filter114. Electrical signal SD is then fed via the corresponding RF cablesection 90 to transmitting antenna 33T, which converts electrical signalSD into a corresponding electromagnetic signal SD″, which then travelsto one or more client devices 46 within the corresponding picocell 40(FIG. 1).

Similarly, receiving antenna 33R receives electromagnetic uplink signalSU″ from one or more client devices 46 within picocell 40 and convertseach such signal to a corresponding electrical signal SU. Thiselectrical travels over the corresponding RF cable section 90 and issignal is fed to RF filter 114, which filters the signal and passes italong to amplifier 110, which amplifies the signal. Electrical signal SUthen travels to bias-T unit 106, which conditions electrical signalSU—i.e., combines a DC signal with the electrical RF signal so it candrive (semiconductor) laser 100 above threshold using a DC currentsource (not shown) and independently modulate the power around itsaverage value as determined by the provided DC current. The conditionedelectrical signal SU then travels to laser 100, which converts theelectrical signal to an corresponding optical signal SU″ that is sent tohead-end unit 20 for processing.

Transponders 30 of the present invention differ from the typical accesspoint device associated with wireless communication systems in that thepreferred embodiment of the transponder has just a fewsignal-conditioning elements and no digital information processingcapability. Rather, the information processing capability is locatedremotely in head-end unit 20. This allows transponder 30 to be verycompact and virtually maintenance free. In addition, the preferredexample embodiment of transponder 30 consumes very little power, istransparent to RF signals, and does not require a local power source, asdescribed below. Moreover, if system 10 needs to be changed (e.g.,upgraded), the change can be performed at head-end unit 20 withouthaving to change or otherwise alter transponders 30.

Example Transponder Configurations

In an example embodiment of transponder 30 such as shown in FIG. 3,dipole antenna system 32 includes one or more antennas 33, such asreceiving antenna 33R and a transmitting antenna 33T. In an exampleembodiment, antennas 33R and 33T are or include respective wiresoriented locally parallel to optical fiber cable 34 (i.e., along they-axis). The ability of dipole antenna system 32 to lie along thedirection of optical fiber cable 34 allows for easy integration of thedipole antenna system into the optical fiber cable relative to othertypes of direction antennas, such as patch antennas. In an exampleembodiment, dipole antenna system 32 includes a circuit-based dipoleantenna, such as available over the Internet from Winizen Co., Ltd.,Kyounggi-do 429-250, Korea.

FIG. 4 is a schematic diagram of an example embodiment of thetransponder 30 of the present invention similar to that shown in FIG. 3,but wherein dipole antenna system 32 includes just a single antenna 33that both transmits and receives electromagnetic signals. In transponder30 of FIG. 4, converter unit 31 includes a RF signal-directing element130, such as a circulator, electrically coupled to single antenna 33 viaa third RF cable section 90.

FIG. 5 is a schematic diagram of an example embodiment of E/O converterunit 60 in converter unit 31, wherein the E/O converter includes anumber of amplifiers 100 electrically connected to photodetector 120.Each amplifier 100 is adapted to amplify a different frequency inelectrical signal SD. This allows for parallel conditioning of differentfrequency bands within transponder 30. A variety of othermulti-frequency amplification and antenna system arrangements are alsopossible for E/O converter 60, as well as for O/E converter 62.

Example Dipole Antenna System Configurations

The transponder 30 of the present invention is capable of supportingnumerous configurations of dipole antenna system 32. FIG. 6 is aschematic diagram of an example embodiment of transponder 30 with adipole antenna system 32 that includes three different antennas 33electrically connected to a power divider 210 via respective RF cablesections 90. Power divider 210 in turn is electrically coupled toconverter unit 31 via a corresponding RF cable section 90.

FIG. 7 is a schematic diagram of an example embodiment of transponder 30with a dipole antenna system 32 similar to that shown in FIG. 6, butthat includes a plurality of power dividers 210 arranged along a RFcable section 90, with each power divider branching off an antenna 33.An advantage of the example embodiment of transponder 30 of FIG. 7 isthat if the optical fiber cable is deployed in a building and oneantenna 33 is obstructed (say, by an air conditioning duct), anotherantenna can still send and receive electromagnetic signals.

FIG. 8 is a schematic diagram of an example embodiment of transponder 30with a dipole antenna system 32 similar to that shown in FIG. 7, whereina portion of the antenna system lies outside of cable coating 58. InFIG. 8, antenna 33 of antenna system 32 is shown arranged outside ofcable coating 58. In an example embodiment, an external covering 220,such as a shrink wrapper, is applied to cable coating 58 to secure andprotect the portion of antenna system 32 that lies outside of the cablecoating.

FIG. 9 is a schematic diagram of an example embodiment of transponder 30with a dipole antenna system 32 similar to that shown in FIG. 8, whereinconverter unit 31 and dipole antenna system 32 are both outside of cablecoating 58 and optionally covered by external covering 220.

FIG. 10 is a schematic diagram of an example embodiment of transponder30 with a dipole antenna system 32 that includes two pairs 234 and 235of wire antennas 33, with each antenna connected to converter unit 33via a corresponding RF cable section 90. Antenna pairs 234 and 235 maybe designed, for example, to transmit and receive at the 5.2 GHz and 2.4GHz frequency bands, respectively (i.e., the IEEE 802 a/b/g standardfrequency bands). The judicious use of RF cable sections 90 in thisexample embodiment mitigates fading and shadowing effects that canadversely affect the dipole radiation pattern 42 and thus the size andshape of picocell 40.

RoF System with Transponder

FIG. 11 is a detailed schematic diagram of an example embodiment ofsystem 10 of FIG. 1, showing the details of an example embodiment ofhead-end unit 20. In an example embodiment, head-end unit 20 includes acontroller 250 that provides electrical RF signals SD for a particularwireless service or application. In an example embodiment, controller250 includes a RF signal modulator/demodulator unit 270 formodulating/demodulating RF signals, a digital signal processor 272 forgenerating digital signals, a central processing unit (CPU) 274 forprocessing data and otherwise performing logic and computing operations,and a memory unit 276 for storing data. In an example embodiment,controller 250 is adapted to provide WLAN signal distribution asspecified in the IEEE 802.11 standard, i.e., in the frequency range from2.4 to 2.5 GHz and from 5.0 to 6.0 GHz. In an example embodiment,controller 250 serves as a pass-through unit that merely coordinatesdistributing electrical RF signals SD and SU from and to outside network24 or between picocells 40.

Head-end unit 20 includes one or more converter pairs 66 each having anE/O converter 60 and an O/E converter 62. Each converter pair 66 iselectrically coupled to controller 250 and is also optically coupled tocorresponding one or more transponders 30. Each E/O converter 60 inconverter pair 66 is optically coupled to an input end 76 of a downlinkoptical fiber 36D, and each O/E converter 62 is optically coupled to anoutput end 74 of an uplink optical fiber 36U.

In an example embodiment of the operation of system 10 of FIG. 11,digital signal processor 272 in controller 250 generates a downlinkdigital RF signal S1. This signal is received and modulated by RF signalmodulator/demodulator 270 to create a downlink electrical RF signal(“electrical signal”) SD designed to communicate with one or more clientdevices 46 in picocell(s) 40. Electrical signal SD is received by one ormore E/O converters 60, which converts this electrical signal into acorresponding optical signal SU′, which is then coupled into thecorresponding downlink optical fiber 36D at input end 76. It is notedhere that in an example embodiment optical signal SD′ is tailored tohave a given modulation index. Further, in an example embodiment themodulation power of E/O converter 60 is controlled (e.g., by one or moregain-control amplifiers, not shown) in order to vary the transmissionpower from dipole antenna system 32, which is the main parameter thatdictates the size of the associated picocell 40. In an exampleembodiment, the amount of power provided to dipole antenna system 32 isvaried to define the size of the associated picocell 40.

Optical signal SD′ travels over downlink optical fiber 36D to an outputend 72 and is processed as described above in connection with system 10of FIG. 2 to return an uplink optical signal SU″. Optical signal SU″ isreceived at head-end unit 20, e.g., by O/E converter 62 in the converterpair 66 that sent the corresponding downlink optical signal SD′. O/Econverter 62 converts optical signal SU′ back into electrical signal SU,which is then processed. Here, in an example embodiment “processed”includes one or more of the following: storing the signal information inmemory unit 276; digitally processing or conditioning the signal incontroller 250; sending the electrical signal SU, whether conditioned orunconditioned, on to one or more outside networks 24 via network links25; and sending the signal to one or more client devices 46 within thesame or other picocells 40. In an example embodiment, the processing ofsignal SU includes demodulating this electrical signal in RF signalmodulator/demodulator unit 270, and then processing the demodulatedsignal in digital signal processor 272.

FIG. 12 is a schematic diagram illustrating a typical prior artpicocellular coverage area 44P formed by a conventional picocellularwireless system that forms symmetric picocells 40P. Note that suchpicocells are traditionally represented as hexagons because they can beshown as tiling a given space without gaps. Picocell coverage area 44Prequires seven optical fiber cables 34P that employ conventionaltransponders having omnidirectional antennas. The conventional opticalfiber cables are optically coupled to a conventional head-end station20P. The dashed-line outer box B in FIG. 12 represents the approximateboundary for picocell coverage area 44P.

FIG. 13 is a schematic diagram of a picocellular coverage area 44 basedon the RoF picocellular wireless system 10 of the present invention thatincludes transponders 30 according to the present invention. The numberof transponders 30 and thus the number of picocells 40 that formpicocellular coverage area 44 are approximately equal to the prior artsystem 10P of FIG. 12. Dashed-line outer box B is provided in FIG. 13 toshow that the size of picocellular coverage area 44 is about the same asthat for picocellular coverage area 44P of FIG. 12. However, eachtransponder 30 of the present invention forms an elongate picocell 40with a long axis A_(P) perpendicular to the local y-direction of opticalfiber cable 34 at the corresponding dipole antenna system 32.Transponders 30 of the present invention thus form a picocellularcoverage area 44 made up of elongate picocells 40 by virtue of orientingthe dipole radiation pattern locally perpendicular to the optical fibercable at the location of each dipole antenna system 32. The elongateshape of picocells 40 allows system 10 to cover substantially the samepicocellular coverage area as area 44P but using only three opticalfiber cables 34—a reduction of over 50% as compared to the optical fibercabling needed for the traditional picocellular coverage area 44P ofFIG. 12. The use of the about the same number of transponders 30 allowsfor picocells 40 operating at the same frequency to be maximallyseparated.

Picocells 40 are elongate because dipole antenna 32 has an asymmetric(elliptical) power distribution in the local x-y plane due to thedifferent power decay rate in the different directions. FIG. 14 is aplot of RF power (dBm) emitted by dipole antenna system 32 vs. thedistance (m) from the antenna along both the x-direction (curve 300) andthe y-direction (curve 302). Curve 302 indicates that along thedirection of the optical fiber cable (y-direction), the decay is fast,so that one can pack transponders 30 more densely along the opticalfiber cable without increasing the picocell-to-picocell crosstalk.

Omnidirectional antennas, such as vertical dipole antennas, typicallyhave a relatively shallow RF power decay rate similar to curve 300 inFIG. 14. Consequently, picocells 40 formed from such antennas are proneto cross-talk. Directive antennas, such as microstrip patches, can havean asymmetric radiation pattern in the x-y plane that can createasymmetric cells. However, these antennas require proper alignment inspace. The dipole antenna system 32 of the present invention producespredictable radiation patterns without any orientation tuning ofindividual antennas. This is because the dipole antenna system 32 isincorporated into (or onto) optical fiber cable 34 in a manner thatallows for the picocell location and orientation to be determined byorienting optical fiber cable 34 rather than orienting individualantennas per se. This makes optical fiber cable 34 easier to manufactureand deploy relative to using other more complex directional dipoleantenna systems.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the present inventionwithout departing from the spirit and scope of the invention. Thus, itis intended that the present invention cover the modifications andvariations of this invention provided they come within the scope of theappended claims and their equivalents.

1. A transponder for a radio-over-fiber (RoF) optical fiber cable,comprising: an electrical-to-optical (E/O) converter; anoptical-to-electrical (O/E) converter; and a dipole antenna systemoperably coupled to the E/O converter and the O/E converter and arrangedrelative to the optical fiber cable so as to create an elongate picocellin a direction locally perpendicular to the optical fiber cable when thetransponder is addressed.
 2. The transponder of claim 1, wherein thedipole antenna includes a transmitting antenna formed from a first wireelectrically coupled to the O/E converter, and a receiving antennaformed from a second wire electrically coupled to the E/O converter,wherein the first and second wires are arranged locally parallel to theoptical fiber cable.
 3. The transponder of claim 1, wherein the opticalfiber cable has an outer coating, and wherein at least a portion of thetransponder resides outside of the outer coating.
 4. The transponder ofclaim 1, wherein the dipole antenna system has includes one or morepower dividers and corresponding one or more antenna elementselectrically coupled to respective power dividers.
 5. The transponder ofclaim 1, wherein the E/O converter and the O/E converter constitute aconverter unit, and wherein the dipole antenna system includes one ormore wires electrically coupled to the converter unit via respective oneor more radio-frequency (RF) cable sections.
 6. A radio-over-fiber (RoF)picocellular wireless system, comprising: a head-end unit adapted tosend and receive optical RF signals; one or more transponders accordingto claim 1; and one or more optical fiber cables that include the one ormore transponders and that optically couple the head-end unit to eachtransponder.
 7. The system of claim 6, wherein each optical fiber cableincludes, for each transponder: a downlink optical fiber opticallycoupled to the head-end unit and to the transponder O/E converter; andan uplink optical fiber optically coupled to the head-end unit and tothe transponder E/O converter.
 8. The system of claim 7, wherein eachoptical fiber cable includes an electrical power line adapted to provideelectrical power to each transponder in the corresponding optical fibercable.
 9. A transponder for forming a picocell as part of aradio-over-fiber (RoF) system having an optical fiber cable opticallyconnected to a head-end unit, comprising: a converter unit adapted toconvert electrical signals to optical signals and vice versa; and adipole antenna system arranged relative to the optical fiber cable so asto create a picocell formed by creating a dipole radiation fielddirected perpendicular to the optical fiber cable at the dipole antennasystem location.
 10. The transponder of claim 9, wherein the dipoleantenna system includes one or more antenna elements each electricallycoupled to the converter unit via corresponding one or moreradio-frequency (RF) cable sections.
 11. The transponder of claim 9,wherein the optical fiber cable includes an outer coating, and whereinat least a portion of the transponder resides outside of the outercoating.
 12. The transponder of claim 11, wherein some or all of thedipole antenna system resides outside of the outer coating.
 13. Aradio-over-fiber (RoF) picocellular wireless system, comprising: one ormore transponders according to claim 9; a head-end unit adapted to sendand receive optical RF signals; one or more optical fiber cables eachhaving at least one transponder and corresponding one or more opticalfiber RF communication links that optically couple the one or moretransponders to the head-end unit; and wherein the one or moretransponders form a picocellular coverage area made up of elongatepicocells formed by each transponder.
 14. The system of claim 13,wherein the head-end unit is adapted to send and transmit optical RFsignals having different frequencies, and the dipole antenna system isadapted to transmit and receive electromagnetic signals having thedifferent frequencies.
 15. The system of claim 13, further including: apower supply operably connected to the head-end unit via an electricalpower line that runs through the one or more optical fiber cables so asto provide electrical power to each transponder.
 16. The system of claim13, wherein each optical fiber cable has an outer coating, and at leasta portion of some or all of the one or more transponders reside outsideof the outer coating.
 17. A method of forming an elongate picocell for aradio-over fiber (RoF) system that includes an optical fiber cable,comprising: transmitting optical RF signals to a transponder via anoptical fiber RF communication link in the optical fiber cable;converting the optical signals to electrical RF signals at thetransponder; converting the electrical signals to electromagnetic RFsignals at the transponder using a dipole antenna system that createsthe elongate picocell in a direction locally perpendicular to theoptical fiber cable.
 18. The method of claim 17, wherein the opticalfiber cable has an outer coating and including providing at least aportion of the dipole antenna system outside of the outer coating. 19.The method of claim 17, including performing the acts therein formultiple transponders so as to form a picocellular coverage area made upof multiple elongate picocells.
 20. The method of claim 17, includingorienting the picocell by orienting the optical fiber cable.